A transition from fluorescent and other forms of traditional lighting to Light Emission Diode (LED) illumination is occurring in various environments including retail outlets, office buildings, warehouses, hospitals, and private homes. LED illumination may provide the benefits of low power consumption, low running cost, long life, and high color rendering effect among other desirable features. Some nations are now moving to ban further manufacture of conventional light bulbs for environmental reasons.
Bar code readers are commonly used in retail environments, including convenience stores, supermarkets and the like. Generally, a laser-scanned barcode reader operates by sweeping a laser beam, commonly having a 650 nm wavelength, over a bar code and receiving light energy reflected from the bar code, which is processed to generate a bar code signal. In a typical application, a laser beam using a 100 Hz scan rate will produce a signal having a frequency range of 30 kHz (kilohertz) to 200 kHz, depending on the resolution of the bar code and the read distance (the distance from the bar code to the bar-code reader).
To suppress power consumption, LED bulbs are generally driven at a frequency within a range of about 30 kHz to 100 kHz which overlaps with the frequencies of many bar code signals. It would be hard for a bar code reader to distinguish light energy from ambient light from light energy from a bar code signal if the frequency ranges of the two signal types overlap. To eliminate interference of the ambient light with bar code readers, U.S. Pat. No. 6,811,087, which is incorporated by reference herein, discloses a technique to scan a bar code using a pulsed laser at a frequency of 2 MHz (megahertz) and using a synchronous detector to detect this frequency and preferably no other frequencies. This technique significantly removes ambient light having a constant intensity (such as sunlight) and light energy from high frequency L.E.D. illumination. However, where there are ambient light frequency components in common with a bar code signal, the decoder within the bar code reader could misread ambient light as being part of a bar code signal, which could lead to a signal reading failure.
Moreover, other possible sources of noise may be present in bar code reading environments as discussed in the following. Laser-scanned bar code readers commonly have exit windows made of glass or plastic (i.e., polycarbonate, Poly-methyl methacrylate material) to protect the sensitive parts inside the reader housing. Although coated with an anti-reflective film, dirt or a finger-print on the exit window would present an optical obstruction resulting in significant back-scatter light being directed toward the photo sensor. The back-scattering of light would be more severe in a retro-reflective type barcode reader, in which the outgoing laser beam and the collected light beam received by the reader share the same optical path. Whereas the signal intensity from a bar code at a distance of 300 to 500 mm has a magnitude of about 0.1 uW (microwatts), the back scatter light could reach a magnitude of 1 uW, which is ten times the magnitude of the bar code signal. The above-described situation may thus lead to an inability of the bar code reader to accurately read a bar code.
Thus, an approach is needed to enable the bar code reader to focus the reading equipment on light energy reflected from the bar code and to screen out light energy from ambient light. One tool for accomplishing this screening process is to employ a synchronous detector that samples at the same rate as the pulse rate of the outgoing laser light.
In existing systems, bar code signals are generally assigned a binary “1” or “0” value right after synchronous detection occurs. Apparatus using synchronous detection is generally less affected by ambient light than other systems, but tend to have difficulty removing low-frequency components of internally scattered light, such as light reflected from a exit window and/or housing, from signal energy from which it is desired to extract a digital bar code signal.
When a signal that includes a substantial low-frequency component is amplified, the signal may acquire a magnitude that exceeds the operating range of the device processing the signal. As a result, the bar code signal components will be collapsed in the ultimate output signal. Even if the output of the synchronous detection is already at the maximum amplitude, the target bar code signal cannot be amplified sufficiently if the low-frequency noise component is large. Consequently, the resulting signal outputs cannot be effectively resolved into logical “1” and logical “0” values, which situation may make it impossible to properly read a bar code.
Accordingly, there is a need in the art for improved systems and methods for removing noise components in bar code reading equipment.